Use hash_newtype macro instead of the hash256 module

We currently implement the `hash256::Hash` type manually however there
is a macro in `bitcoin::hashes` that can do that for us.

For context, the `hash256::Hash` type exists solely to enable the type
to display forwards since `hashes::sha256d` displays backwards. The
`hash_newtype` macro takes an argument to configure display.

Author: tcharding

src/miniscript/hash256.rs

@@ -27,7 +27,6 @@ pub mod analyzable;
 pub mod astelem;
 pub(crate) mod context;
 pub mod decode;
-pub mod hash256;
 pub mod iter;
 pub mod lex;
 pub mod limits;
@@ -459,6 +458,18 @@ impl_from_str!(
 
 serde_string_impl_pk!(Miniscript, "a miniscript", Ctx; ScriptContext);
 
+/// Provides a Double SHA256 `Hash` type that displays forwards.
+pub mod hash256 {
+    use bitcoin::hashes::{hash_newtype, sha256d};
+    hash_newtype!(
+        Hash,
+        sha256d::Hash,
+        32,
+        doc = "A bitcoin block hash.",
+        false
+    );
+}
+
 #[cfg(test)]
 mod tests {